Earlier work in detecting contraband substances centered on the subject of nuclear magnetic resonance (NMR). Work in this area is reflected in U.S. Pat. Nos. 4,166,972, 4,296,378 and 4,514,691. A drawback of NMR is that is requires relatively large magnets. Magnets are relatively expensive, would likely cause personnel to be exposed to large static magnetic fields, and could damage magnetically recorded material.
Another attempt at explosives detection employed thermal neutron analysis (TNA), which can detect nitrogen in any form. Although it could detect explosives, it was also triggered by nitrogen-rich nylon and wool, and other innocuous items. These shortcomings resulted in a high rate of false positives. Because it employed potentially hazardous radioactive emissions, TNA systems were also required to be heavily shielded. As a consequence, TNA systems were very large, very expensive, and also produced a high rate of false positives.
X-ray screening, commonly used in airports, does not have the same overall limitations as TNA. However, it cannot alert the operator to the presence of explosives or drugs, much less identify them. X-ray screening can only "see images that the operator must interpret quickly." Further, X-ray screening emits potentially hazardous ionizing radiation.
With respect to explosives, plastic explosives such as C4 and Semtex, containing RDX and PETN, have an almost infinite variety of possible shapes and uses for terrorist bombing tactics. Plastic explosives are highly stable, have clay-like malleability and are deadly in relatively small quantities. A small piece of plastic explosive, a detonator, and a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately, without close--and potentially dangerous--visual inspection, plastic explosives can be made virtually untraceable. Because of the drawbacks of TNA, NMR and X-ray, as mentioned above, they have generally proven ineffective for practical bulk detection of these types of explosives. In particular, detection of sheet explosives, typically having a thickness as small as 6.35 mm (0.25 inch), has not been effectively accomplished by prior technologies.
The wide-scale attempts to fight the illegal drug trade indicates that narcotics detection is also extremely important. The need for a simple procedure for detecting drugs inside sealed containers, mail parcels, and other small packages, quickly and accurately, is immeasurable. Conventional detection methods are time-consuming, costly, and have only marginal reliability at best.
NQR is a branch of radio frequency spectroscopy that exploits the inherent electrical properties of atomic nuclei. Nuclei with non-spherical electric charge distributions possess electric quadrupole moments. Quadrupole resonance arises from the interaction of the nuclear quadrupole moment of the nucleus with the local applied electrical field gradients produced by the surrounding atomic environment.
Any chemical element's nucleus which has a spin quantum number greater than one half can exhibit quadrupolar resonance. Many substances (approximately 10,000) have been identified that exhibit quadrupolar resonance, among such nuclei being: .sup.7 Li, .sup.9 Be, .sup.14 N, .sup.17 O, .sup.23 Na, .sup.27 Al, .sup.35 Cl, .sup.37 Cl, .sup.39 K, .sup.55 Mn, .sup.75 As, .sup.79 Br, .sup.81 Br, .sup.127 I, .sup.197 Au, and .sup.209 Bi. It so happens that some of these quadrupolar nuclei are present in explosive and narcotic materials, among them being nitrogen (.sup.14 N), chlorine (.sup.35 Cl, .sup.37 Cl), oxygen (.sup.17 O), sodium (.sup.23 Na), and potassium (.sup.39 K). The most studied quadruple nucleus for explosives and narcotics detection is nitrogen.
In solid materials, electrons and atomic nuclei produce electric field gradients. These gradients modify the energy levels of any quadrupolar nuclei, and hence their characteristic transition frequencies. Measurements of these frequencies or relaxation time constants, or both, can indicate not only which nuclei are present but also their chemical environment.
When an atomic quadrupolar nucleus is within an electric field gradient, variations in the local field associated with the field gradient affect different parts of the nucleus in different ways. The combined forces of these fields cause the quadrupole to experience a torque, which causes it to precess about the electric field gradient. Precessional motion generates an oscillating nuclear magnetic moment. An externally applied radio frequency (RF) magnetic field in phase with the quadrupole's precessional frequency can tip the orientation of the nucleus momentarily. The energy levels are briefly not in equilibrium, and immediately begin to return to equilibrium. As the nuclei return, they produce an RF signal, known as the free induction decay (FID). A pick-up coil detects the signal, which is subsequently amplified by a sensitive receiver to measure its characteristics.
One distinguishing feature of an NQR response is its precessional frequency. Two independent factors determine the precessional frequency: the quadrupolar nucleus, and its local crystalline environment. There may be one or more characteristic NQR frequencies for each substance containing quadrupolar nuclei.
The second distinguishing features are the NQR relaxation times. Relaxation times are a measure of the nuclei's rate of return to the equilibrium state following disturbance by an RF pulse. Relaxation times are compound-, temperature-, and pressure-specific. Relaxation times also determine the repetition rate and timing of RF pulses required for exciting and detecting a specific NQR signal. Relaxation times can be as short as a few hundred microseconds or as long as several seconds.
Detection of NQR signals normally requires RF transmitting and receiving apparatus. To minimize noise and radio frequency power requirements and improve receiver sensitivities, conventional NQR systems use a narrow band (high Q) sample coil in both the transmitting and receiving equipment. Even so, several factors can significantly degrade the effectiveness of detecting NQR signals. Among these factors are: (1) the presence of conductive materials inside the sample coil; (2) the presence of materials with a high dielectric constant inside the sample coil; (3) temperature, which can affect the value of the capacitance used for tuning and matching the RF coil; and (4) mechanical movement of the coil which respect to its surroundings. All of these factors can cause serious de-tuning of the detection apparatus, which in turn, lowers the detection sensitivity of the coil. Accordingly, NQR systems have largely been limited to small sample laboratory systems with little or no "real-world" potential.
The NQR energy level transitions are observed primarily in the radio frequency range. Detection of these transitions requires an RF source to excite the transition, and an RF receiving mechanism to detect the signals returning from the nuclei. Normally, the signals appear at a pre-defined frequency. An RF coil tuned to, or close to, that predefined frequency can excite and/or detect those signals. The signals are of very low intensity and can only be observed for a short time, approximately 10 .mu.s to 10 ms. As a consequence, there is a need for an NQR receiver that can be tuned to a (usually) high Q, has very low noise, and is capable of fast recovery after a high-voltage RF pulse.
Previous work in this area is reflected in U.S. Pat. Nos. 4,887,034, 5,206,592, 5,233,300 and 5,365,171. Use of NQR for explosives and narcotics detection is also discussed in Buess et al., Explosives Detection By .sup.14 N Pure NQR, Advances in Analysis and Detection of Explosives (J. Yinon (ed.)) pp. 361-368 (1993), and Shaw, Narcotics Detection Using Nuclear Quadrupole Resonance (NQR), Contraband and Cargo Inspection Technology International Symposium, Washington, D.C., pp 333-341 (1992).
Detection by means of NQR is possible for both explosives and narcotics, partially because they have as a constituent element .sup.4 N in crystalline form. Particularly with respect to narcotics, this is true of cocaine base, cocaine hydrochloride and heroin based narcotics. The hydrochloride forms of narcotics, such as cocaine hydrochloride, also contain quadrupolar nuclei .sup.35 Cl and .sup.37 Cl. For example, U.S. Pat. No. 5,206,592 discloses the concept at using a planar meanderline coil past which a specimen is passed. The coil is tuned to about the target signal frequency and applies a pulsed RF signal to the specimen and picks up the nuclei relaxation signals from target substances present in the specimen. A CPU is employed to process the received signals and to activate an alarm when the received signal exceeds a predetermined threshold.
Where coils are involved, the Q of the coil is often of major importance. Ochi et al., Analysis of a Magnetic Resonance Imaging Antenna Inside an RF Shield, Electronics and Communications in Japan, Part 1, Vol. 77, No. 1, pp 37-45 (1994), teach how to quantify the change in Q of an MRI antenna with changes in length and diameter of the shield. However, this deals with MRI and not NQR, and is relevant only to humans and not to packages or baggage. An automatic tuning system is disclosed in Butler et al., High-Power Radio frequency Irradiation system with Automatic Tuning, Rev. Sci. Instrum., Vol. 53, No. 7, pp 984-988 (1982). This tuning system is useful in an NQR spectrometer and in other nuclear resonance experiments involving frequency sweeps. In the Butler system, data is predetermined for various frequencies and is not designed to compensate for an unknown coil loading. U.S. Pat. No. 5,209,537 discloses a method for matching antennas in an NMR imaging apparatus for use in producing tomograms.
A significant factor in contraband detection by means of NQR is that quadrupolar nuclei that are commonly present, and potentially readily observable, in narcotics and explosives include nitrogen (.sup.14 N) and chlorine (.sup.35 Cl and .sup.37 Cl), among other possible nuclei. Thus, in commercial applications it is necessary to be able to detect quadrupolar nuclei contained within articles of mail, mail bags or airline baggage, including carry-on and checked luggage. While the resonant frequencies of the nitrogen in these substances differs for each chemical structure, these resonant frequencies are well defined and consistent. By applying an RF signal to a container having any of these suspected substances inside, and then detecting any quadrupolar resonance thus engendered by the application of RF pulses, the identity of the contraband substance can be easily determined.